4 key qtouch™ sensor ic

7/29/2019 4 KEY QTOUCH SENSOR IC

1/12

lQ QT2 4 0 -ISSG

4 KEYQTOUCH SENSOR IC

APPLICATIONS

! Instrument panels! Gaming machines

! Access systems! Pointing devices

! Appliance controls! Security systems

! PC Peripherals! Backlighted buttons

The QT240 charge-transfer (QT) QTouch IC is a self-contained digital sensor IC capable of detecting near-proximity or touchon four electrodes. It allows electrodes to project independent sense fields through any dielectric like glass, plastic, stone,ceramic, and wood. It can also turn metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch.This capability coupled with its continuous self-calibration feature can lead to entirely new product concepts, adding high value

to product designs.Each of the channels operates independently of the others, and each can be tuned for a unique sensitivity level by simplychanging its sample capacitor value. Two speeds are supported, one of which consumes only 90A of typical current at 4V.Unique among capacitance sensors, the device incorporates spread -spectrum modulation for unsurpassed EMC compliance.

The devices are designed specifically for human interfaces, like control panels, appliances, gaming devices, lighting controls,or anywhere a mechanical switch or button may be found; they may also be used for some material sensing and controlapplications.

These devices feature a SYNC pin which allows for synchronization with additional similar parts and/or to an external source tosuppress interference. This pin doubles as a drive pin for spread-spectrum modulation. Option pins are provided which allowdifferent timing and feature settings.

The RISC core of these devices use signal processing techniques pioneered by Quantum which are designed to survivenumerous real-world challenges, such as stuck sensor conditions, component aging, moisture films, and signal drift.

By using the charge-transfer principle, these devices deliver a level of performance clearly superior to older technologies yetare highly cost-effective.

LQ Copyright 2003-2006 QRG LtdQT240R R1.11/1006

SNS1

1

2

3

4

5

6

7

8

9

11

12

13

14

15

16

17

18

SNS1K

OUT1

OUT2

VSS

OUT3

SS/SYNC

n.c.

OSC

OUT4

/RES

VDD

SNS4

SNS4K

SNS3K

SNS3SNS2K

QT240

20-SSOP

SNS2

10

19

20

n.c. n.c.

" Four independent charge-transfer (QT) touch keys

" Individual outputs per channel - active high

"

Projects prox fields through any dielectric" Sensitivity easily adjusted on a per-channel basis

" 100% autocal for life - no adjustments required

" 3.9V ~ 5.5V single supply operation

" 10s, 60s, infinite autorecal timeout (strap options)

" Sync pin for line sync to suppress noise

" Spread spectrum operation

" Pin options for auto recalibration timings

" Extremely low cost per key

" 20-SSOP RoHS compliant package

QT240-ISSG-40

o

C to +85

o

C

SSOP-20TA

AVAILABLE OPTIONS


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62.3 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.7 Fast Positive Recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.6 Forced Sensor Recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.5 Detection Integrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.4 Max On-Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.3 Threshold Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1.2 Drift Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1 Signal Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42 QT240 Specifics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.4.4 Key Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.4.3 Decreasing Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.4.2 Alternative Ways to Increase Sensitivity . . . . . . . . . . . . . . . . . . . . . . . .41.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.4 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31.3 Electrode Drive; Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31.2 Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

114.8 Moisture Sensitivity Level (MSL) . . . . . . . . . . . . . . . . . . . . . . . . . . . .114.7 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104.6 Mechanical Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104.5 Signal Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94.4 DC Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94.3 AC Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .94.2 Recommended Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . .94.1 Absolute Maximum Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . .94 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.6 Crosstalk Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.5 ESD Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.4 Unused Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.3 Oscillator; Spread Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73.2 Power Supply, PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63.1 Cs Sample Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 Circuit Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contents


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1 Overvi ew

1.1 IntroductionQT240 devices are burst mode digital charge-transfer (QT)sensor ICs designed specifically for touch controls; theyinclude all hardware and signal processing functionsnecessary to provide stable sensing under a wide variety ofconditions. Only a single low cost capacitor per channel is

required for operation.Figures 1.1 and 1.2 show basic circuits for these devices.See Table 1.1 for device pin listings.

The devices employ bursts of charge-transfer cycles toacquire signals. Burst mode permits low power operation,dramatically reduces RF emissions, lowers susceptibility toRF fields, and yet permits excellent speed. Internally, signalsare digitally processed to reject impulse noise using a'consensus' filter that requires six consecutive confirmationsof detection.

The QT switches and charge measurement hardwarefunctions are all internal to the device. A single-slopeswitched capacitor ADC includes the QT charge and transferswitches in a configuration that provides direct ADCconversion; an external Cs capacitor accumulates the chargefrom sense-plate Cx, which is then measured.

Larger values of Cx cause the charge transferred into Cs torise more rapidly, reducing available resolution; as aminimum resolution is required for proper operation, this canresult in dramatically reduced gain. Larger values of Csreduce the rise of differential voltage across it, increasingavailable resolution by permitting longer QT bursts. Thevalue of Cs can thus be increased to allow larger values ofCx to be tolerated. The IC is responsive to both Cx and Cs,and changes in either can result in substantial changes insensor gain.

Unused channels: If a channel is not used, a dummy sense

capacitor (nominal value 1nF) of any type plus a 2.2K seriesresistor must be connected between unused SNS pin pairs,to ensure correct operation.

1.2 Operating ModesThe QT240 features spread-spectrum acquisitioncapability, external synchronization of acquirebursts, and fast and slow acquisition modes. Thesemodes are enabled via high-value resistorsconnected to the SNS pins to ground or Vdd. Theseresistors are required in every circuit.

There are two basic modes as shown in Figures 1.1and 1.2.

Low-power Sync mode: In this mode the deviceoperates with about a 100ms response time andvery low current (about 90A average at 4.0V). Thismode allows the device to be synchronized to anexternal clock source, which can be used to eithersuppress external interference (such as from50/60Hz wiring) or to decrease response time(which will also increase power consumption).Spread-spectrum operation is not directly supportedin this mode. Sync usage is optional; the Sync pinshould be grounded if unused.

Fast, Spread-Spectrum mode: In this mode the deviceoperates with ~40ms response times but higher current drain(~1.5mA @ 4.0V). This mode also supports spread-spectrumoperation via a few optional passive parts (if desired). Syncoperation is not supported in this mode.

1.3 Electrode Drive; WiringThe QT240 has four completely independent sensingchannels. The conversion process treats Cs on each channelas a floating transfer capacitor; as a direct result, senseelectrodes can be connected to either SNS pin and thesensitivity and basic function will be the same; howeverelectrodes should be connected to SNSnK lines to reduceEMI susceptibility.

The PCB traces, wiring, and any components associatedwith or in contact with either SNS pin will become touchsensitive and should be treated with caution to limit the toucharea to the desired location.

lQ 3 QT240R R1.11/1006

Figure 1.1 Low Power, Synchronized Circuit

S1

S3

S2

VDD

KEY2

KEY3

10nF

CS1

KEY1

VDD

VDD62K R4

10nF

CS3

10nF

CS4

2.2K

RS3

KEY4

VDD

1M

R2

22KRSNS1

2.2K

RS1

22KRSNS4

2.2K

RS4

OPT1

OUT1

SYNC

OUT4

OUT2

OUT3

10nF

CS2

RS2

2.2K

1M

R1

VDD

1M

R3

10 secondtimeout shown

SPEEDOPT

22KRSNS2

OPT2

22KRSNS3

QT240-ISS

Sense pin (to Cs3, electrode); OPT1SNS3K20

Sense pin (to Rs3 + Cs3)SNS319

Sense pin (to Cs4, electrode); OPT2SNS4K18


Ground or no connectVSS16

Oscillator bias inOSC15

Power: +4.0 to +5V locally regulatedVDD14

Reset pin, active low. Can usually tie to Vdd./RES13

Output, key 4OUT412

Unbonded internallyn.c.11

Unbonded internallyn.c.10

Sync in and/or spread-spectrum driveSYNC/SS9

GroundVSS8 Output, key 3OUT37

Output, key 2OUT26

Output, key 1OUT15

Sense pin (to Cs1, electrode); speed optionSNS1K4


Sense pin (to Cs2, electrode)SNS2K2


DescriptionNamePin

Table 1.1 Pin Listing - QT240-ISS


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Multiple touch electrodes connected to any SNSnK can beused, for example, to create control surfaces on both sides ofan object.

It is important to limit the amount of stray capacitance on theSNS terminals, for example by minimizing trace lengths andwidths to allow for higher gain without requiring higher valuesof Cs. Under heavy delta-Cx loading of one key, crosscoupling to another keys trace can cause the other key totrigger. Therefore, electrode traces from adjacent keysshould not be run close to each other over long runs in order

to minimize cross-coupling if large values of delta-Cx areexpected, for example when an electrode is directly touched.This is not a problem when the electrodes are workingthrough a plastic panel with normal touch sensitivity.

1.4 Sensitivity

1.4.1 IntroductionSensitivity can be altered to suit various applications andsituations on a channel-by-channel basis. The easiest andmost direct way to impact sensitivity is to alter the value ofeach Cs; more Cs yields higher sensitivity. Each channel hasits own Cs value and can therefore be independentlyadjusted.

1.4.2 Alternative Ways to IncreaseSensitivity

Sensitivity can also be increased by using biggerelectrode areas, reducing panel thickness, or usinga panel material with a higher dielectric constant .

1.4.3 Decreasing SensitivityIn some cases the circuit may be too sensitive.Gain can be lowered further by a number of

strategies: a) making the electrode smaller, b)making the electrode into a sparse mesh using ahigh space-to-conductor ratio, or c) by decreasingthe Cs capacitors.

1.4.4 Key BalanceA number of factors can cause sensitivityimbalances. Notably, SNS wiring to electrodes canhave differing stray amounts of capacitance toground. Increasing load capacitance will cause adecrease in gain. Key size differences, andproximity to other metal surfaces can also impactgain.

The four keys may thus require balancing to

achieve similar sensitivity levels. This can be bestaccomplished by trimming the values of the fourCs capacitors to achieve equilibrium. The four Rsresistors have no effect on sensitivity and should

not be altered. Load capacitances can also be added tooverly sensitive channels to ground, to reduce their gains.These should be in the order of a few picofarads.

2 QT240 Spec i f ic s

2.1 Signal Processing

2.1.1 IntroductionThese devices process all signals using 16 bit math , using anumber of algorithms pioneered by Quantum. Thesealgorithms are specifically designed to provide for highsurvivability in the face of adverse environmental changes.

2.1.2 Drift CompensationSignal drift can occur because of changes in Cx , Cs, andVdd over time. If a low grade Cs capacitor is chosen, thesignal can drift greatly with temperature. If keys are subjectto extremes of temperature or humidity, the signal can alsodrift. It is crucial that drift be compensated, else falsedetections, nondetections, and sensitivity shifts will follow.

Drift compensation (Figure 2.1) is a method that makes the

reference level track the raw signal at a slow rate, only whileno detection is in effect. The rate of referenceadjustment must be performed slowly elselegitimate detections can also be ignored. The ICdrift compensates each channel independentlyusing a slew-rate limited change to the referencelevel; the threshold and hysteresis values areslaved to this reference.

Once an object is sensed, the drift compensationmechanism ceases since the signal is legitimatelyhigh, and therefore should not cause the referencelevel to change.

lQ 4 QT240R R1.11/1006

Figure 1.2 Fast, Spread-spectrum Circuit

S1

S3

S2

VDD

KEY

2

KEY

3

OUT4

OUT1

OUT2

OUT3

10nF

CS1

RS1

2.2K

KEY

1

C1

22nF

VDD

10nF

CS3

10nF

CS4

RS3

2.2K

KEY

4

VDD

VDD

1M

R2

22KRSNS1

360KR5

22KRSNS4

RS4

2.2K

OPT1

10nF

CS2

RS2

2.2K1M

R1

R6180K

VDD

62K R4

1M

R3

10 secondtimeout shown

22KRSNS2

SPEEDOPT

OPT2

22KRSNS3

QT240_ISS

Figure 2.1 Drift Compensation

Threshold

SignalHysteresis

Reference

Output


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The signal drift compensation is 'asymmetric'; the referencelevel drift-compensates in one direction faster than it does inthe other. Specifically, it compensates faster for decreasingsignals than for increasing signals. Increasing signals shouldnot be compensated for quickly, since an approaching fingercould be compensated for partially or entirely before evenapproaching the sense electrode. However, an obstructionover the sense pad, for which the sensor has already madefull allowance, could suddenly be removed leaving the

sensor with an artificially elevated reference level and thusbecome insensitive to touch. In this latter case, the sensorwill compensate for the object's removal very quickly, usuallyin only a few seconds.

With large values of Cs and small values of Cx, driftcompensation will appear to operate more slowly than withthe converse.

Drift Compensation in Slow Mode: Drift compensationrates in Slow mode are preserved if there is no Sync signal,and the rates are derived from the ~90ms Sleep interval.However if there is a Sync signal, then drift compensationrates are derived from an assumption that the Syncperiodicity is ~18ms (which is corresponds to 55.5Hz). Thus,drift compensation timings in Sync mode are correct for an

~18ms Sync period but different (slower or faster) for otherSync periods. For example a Sync period of 36ms wouldhalve the expected drift compensation rates.

2.1.3 Threshold LevelThe internal threshold level is fixed at 12 counts for all fourchannels. The hysteresis is fixed at 2 counts (17%).

2.1.4 Max On-DurationIf a sufficiently large object contacts a key for a prolongedduration, the signal will trigger a detection output preventingfurther normal operation. To cure such stuck key conditions ,the sensor includes a timer on each channel to monitordetection duration. If a detection exceeds the maximum timer

setting, the timer causes the sensor to perform a fullrecalibration (if not set for infinite) . This is known as the MaxOn-Duration feature.

After the Max On-Duration interval, the sensor channel willonce again function normally, even if partially or fullyobstructed, to the best of its ability, given electrodeconditions. There are three timeout durations available viastrap option: 10s, 60s, and infinite (Table 2.2).

Max On-Duration works independently per channel; atimeout on one channel has no effect on another channel.Note also that the timings in Table 2.2 are dependent on theoscillator frequency in Fast mode. Doubling therecommended frequency will halve the timeouts. This is nottrue in Slow mode.

Infinite timeout is useful in applications where a prolongeddetection can occur and where the output must reflect thedetection no matter how long. In infinite timeout mode, thedesigner should take care to ensure that drift in Cs, Cx, andVdd do not cause the device to stick on inadvertently evenwhen the target object is removed from the sense field.

Timeouts are approximate and can vary substantially overVdd and temperature, and should not be relied upon forcritical functions. Timeouts are also dependent on operatingfrequency in Fast mode.

Max On-Duration in Slow Mode: When Sync mode is usedin Slow mode, the Max On-Duration timings are derived fromthe Sync period. The device assumes the Sync periodicity is18ms (midway between 50Hz and 60Hz sync timings). Thus,Max On-Duration timings in Sync mode are correct for an18ms Sync period but different (shorter or longer) for otherSync periods. For example a Sync period of 36ms woulddouble all expected Max On-Duration timings.

2.1.5 Detection IntegratorIt is desirable to suppress false detections due to electricalnoise or from quick brushes with an object. To this end,these devices incorporate a per-key Detection Integratorcounter that increments with each signal detection exceedingthe signal threshold (Figure 2.1) until a limit count is reached,after which an Out pin becomes active. If a no detect issensed even once prior to the limit, this counter is reset tozero and no detect output is generated. The required limitcount is 6.

The Detection Integrator can also be viewed as a'consensus' vote requiring a detection in successive samplesto trigger an active output.

In Slow mode, the detect integrator forces the device to

operate faster to confirm a detection. The six successiveacquisitions required to affirm a detection are done withoutbenefit of a low power sleep mode between bursts.

2.1.6 Forced Sensor RecalibrationPin 13 is a Reset pin, active-low, which in cases wherepower is clean can be simply tied to Vdd. On power-up, thedevice will automatically recalibrate all channels of sensing.

Pin 13 can also be controlled by logic or a microcontroller toforce the chip to recalibrate, by toggling it low for 10 s ormore, then raising it high again.

2.1.7 Fast Positive Recalibration

If the sensed capacitance becomes lower by 5 counts thanthe reference level for 2 seconds, the sensor will considerthis to be an error condition and will force a recalibration onthe affected channel.

2.2 OptionsThese devices are designed for maximum flexibility and canaccommodate most popular sensing requirements via optionpins.

The option pins are read on power-up and about once every10 seconds while the device is not detecting touch on anychannel. Options are set using high value resistorsconnected to certain SNS pins, to either Vdd or Vss. Theseoptions are read 25 times over 250s to ensure that they are

not influenced by noise pulses. All 25 samples must agree.However, large values of Cx on the SNS wires can loaddown the pins to the point where the 1M pull-up resistorscannot pull high fast enough, and the pins are readerroneously as a result. Cx should be below 50pF to preventerrors; this value can be read with a conventionalcapacitance meter with the QT240 removed.

The option setting resistors are mandatory and cannot bedeleted. They must be strapped to either Vdd or Vss.

lQ 5 QT240R R1.11/1006


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Speed option (Strap S1): This jumper selects whether thedevice acts in a slower, low power mode with a responsetime of approximately 100ms, or in a Fast mode with aresponse time of 40ms typical. Fast mode consumes

substantially more power than the Slow mode, but alsoenables the use of spread-spectrum detection. Only Slowmode supports the use of external Sync (Section 2.3).

Response time can also be modified by changing theoscillator frequency (Section 3.3).

Recalibration / toggle select (S2, S3): See Table 2.2.There are three recalibration timing options (MaxOn-Duration, see Section 2.1.4) and one toggle modeoption. The recalibration options control how long it takes fora continuous detection to trigger a recalibration on a key.When such an event occurs, only the stuck key isrecalibrated. S2 / S3 should be connected as shown inTable 2.2 to achieve the desired Max On-Duration of either10s, 60s, or infinite.

Toggle option: One option is toggle mode, which allows allfour keys to behave with flip-flop action. In this mode, eachkeys corresponding OUT pin will toggle High / Low withsuccessive touches on the key. The underlying MaxOn-Duration is 10s in this mode. If a timeout occurs inToggle mode, the toggle state is not affected. Toggle stateflips only when the corresponding Out pin goes High.

This is useful for controlling power loads, for example inkitchen appliances, power tools, light switches, etc . orwherever a touch-on / touch-off effect is required.

2.3 SynchronizationSync capability is only present in Slow mode (Section 2.2). If

SYNC is not desired, SYNC/SS should be connected to Vss.

Adjacent capacitive sensors that operate independently cancross-interfere with each other in ways that will createsensitivity shifts and spurious detections. Since QuantumsQT devices operate in burst mode, the opportunity exists tosolve this problem by time-sequencing sensing channels sothat physically adjacent keys do not sense at the same time.Within the QT240 the four channels operate synchronously,so it is not possible for these channels to cross interfere.However, two or more adjacent chips will cross-interfere ifthey are not synchronized to each other. The same is true ofthe effects of unsynchronized external noise sources.

External noise sources can also be heavily suppressed bysynchronizing the QT240 to the noise source period. Externalnoise creates an aliasing or beat frequency effect betweenthe sampling rate of the QT part and the external noisefrequency. This shows up as a random noise component onthe internal signals, which in turn can lead to false activation.

Mains frequency is one common source of interference. Asimple AC zero-crossing detector feeding the SYNC pin isenough to suppress this kind of periodic noise. Multiple

devices tied to SYNC can be synchronized to the mainsfrequency in this fashion.

If two physically adjacent devices are to be synchronized toeach other, they should be connected via the SYNC pin to aclock source that is slower than the burst rate of eitherdevice. For example, a 50Hz clock can synchronize twoQT240s running with burst spacings of up to 10ms each.The two QT240s should be synchronized on oppositephases of the clock source, i.e. the clock source should feedone part and its inverted phase, the other part.

A sync pulse on SYNC/SS in Slow mode acts to break theQT240 out of its sleep state between bursts, and to doanother burst. The device will then go back to sleep againand await a new SYNC pulse. If a Sync pulse does not arrivewithin about 90ms, it will wake again and run normally.

External sync pulses can be used to accelerate responsetime (at the expense of power) in Slow mode. Sync pulsesrunning at 25Hz for example will improve response time by afactor of 2. Sync cannot be used to slow down the device.

Sync Mode Effects on Timings: In the absence of a Syncsignal, the Max On-Duration timings and drift compensationrates in Slow mode are nominally correct. It should beunderstood that the Max On-Duration timings and driftcompensation rates are slaved to the burst interval in Slowmode, and that changing the burst interval will have directeffects on these parameters.

Since the most common use of Sync is to synchronize thedevice to Mains frequency (50 or 60Hz) the device makes anassumption that the presence of a Sync signal is at 55Hz,and the timings are made to be correct at this frequency.

Should the Sync pulses vary from this frequency, the MaxOn-Duration timings and drift compensation rates will varyproportionately. Thus, if the Sync pulses are 25Hz, the10-second Max On-Duration timing will become 10*55/25 =22 seconds nominal. Only at Sync=55Hz will the 10s timeoutbe 10s (the same as if there were no Sync signal, or thedevice was in Fast mode).

3 Circ ui t Guidel ines3.1 Cs Sample CapacitorCharge sampler capacitors Cs can be virtually any plasticfilm or low to medium-K ceramic capacitor. The normal Csrange is 4.7nF to 47nF depending on the sensitivity required;larger values of Cs require higher stability to ensure reliablesensing. Acceptable capacitor types for most uses includeplastic film (especially PPS film and polypropylene film) andX7R ceramic. Lower grades than X7R are not advised;higher-K ceramics have nonlinear dielectrics which induceinstabilities.

lQ 6 QT240R R1.11/1006

VddSlow / Sync

VssFast / Spread Spectrum

Table 2.1 S1 Speed / Sync Options - SNS1K Pin 4

Timings assume 100 kHz operation

infiniteVssVssDC Out

10sVddVddToggle

60sVssVddDC Out

10sVddVssDC OutMax On-Duration

S3

SNS4K pin 18

S2

SNS3K pin 20

Table 2.2 OPT Options


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3.2 Power Supply, PCB LayoutThe power supply can range from 3.9 to 5.5 volts. If thisfluctuates slowly with temperature, the device will track andcompensate for these changes automatically with only minorchanges in sensitivity. If the supply voltage drifts or shift squickly, the drift compensation mechanism will not be able tokeep up, causing sensitivity anomalies or false detections.The QT240 will track slow changes in Vdd, but can beseriously affected by rapid voltage steps.

If the supply is shared with another electronic system, careshould be taken to assure that the supply is free of digitalspikes, sags, and surges which can cause adverse effects.

The supply is best locally regulated using almost anythree-terminal LDO device from 4.0V to 5V.

For proper operation a 0.1F or greater bypass capacitormust be used between Vdd and Vss; the bypass capacitorshould be placed very close to the device Vss and Vdd pins.

The PCB should, if possible, include a copper pour underand around the IC, but not extensively under the SNS lines.

3.3 Oscillator; Spread Spectrum

The oscillator is an internal type using an external currentbias source. Normal operation occurs at ~100 kHz 20% withR=62K at Vdd = 4.0V.

In Fast mode the oscillator can be spread-spectrummodulated with a simple external RC network to avoiddwelling on any one frequency. The circuit works by currentmodulating the oscillator bias to provide a chirp modulationwithin each burst. This helps dramatically with both RFemissions and susceptibility. The circuit for this is shown inFigure 1.2. The SYNC/SS pin outputs a rectangular pulsewith a period of one burst. At the end of the burst, SYNC/SSclamps to ground. This forms a sawtooth modulationwaveform to create the frequency chirp modulation(Figure 3.1).

The detection integrator (DI) filters out false detectionscaused by interference on up to any six of these acquisitions.The DI is a consensus filter that throws out a pendingdetection if even one of the six samples does not confirm adetection. As a result of this feature, it is extremely difficultfor an external signal to interfere with the device.

The typical modulation band of this circuit is 7% around thecenter frequency.

The oscillator frequency can be measured byobserving the acquisition pulses on any sensechannel with an oscilloscope. The first twopositive pulses in each burst will be exactly10s from rising edge to rising edge (100kHz) if

the oscillator is set correctly (no spreadspectrum). Some subsequent pulse pairs willhave wider spacings; this is normal.

If desired the response time of the device inFast mode can be modified by altering theoscillator frequency. The device can be set toany center frequency from 20kHz to 150kHz bysimply altering the value of the bias resistor.Lower values of R will speed up the device.Higher resistor values will slow down the deviceand reduce power consumption.

Slow mode relies on an internal RC timer whichcannot be adjusted via the bias resistor. Duringintervals between bursts in Slow mode, the

100kHz oscillator is disabled.

Spread Spectrum in Slow Mode: The QT240 does notsupport spread spectrum in Slow mode directly via theSYNC/SS pin. However, the designer can still implementspread spectrum by modulating the oscillator through aresistor to OSC with a triangle or sawtooth wave.

The modulation signal should be exactly synchronous witheach burst, so that the resulting acquired signal is modulatedin the same way. If this rule is not followed, the signals willcontain modulation noise and false detections may occur.

3.4 Unused ChannelsUnused SNS pins should not be left open. They should havea small-value noncritical dummy Cs capacitor plus a 2.2Kseries-R connected to their SNS pins to allow the internalcircuit to continue to function properly. A nominal value of1nF (1,000pF) X7R ceramic will suffice.

Unused channels should not have sense traces orelectrodes connected to them except for the required optionresistors, which must always be installed and connected toVdd or Vss.

3.5 ESD ProtectionNormally, only a series resistor is required for ESDsuppression. A 22K Rsns resistor in series with the sensetrace is sufficient in most cases. The dielectric panel (glassor plastic) usually provides a high degree of isolation toprevent ESD discharge from reaching the circuit. Rsnsshould be placed close to the chip.

If the Cx load is high, Rsns can prevent total charge andtransfer and as a result gain can deteriorate. If a reduction inRsns increases gain noticeably, the lower value should beused. Conversely, increasing the Rsns can result in addedESD and EMC benefits provided that the increase does notdecrease sensitivity.

3.6 Crosstalk PrecautionsAdjacent sense traces might require intervening groundtraces in order to reduce capacitive cross bleed if highsensitivity is required or high values of delta-Cx areanticipated (for example, from direct human touch to anelectrode connection). In normal touch applications behindplastic panels this is almost never a problem regardless ofhow the electrodes are wired.

lQ 7 QT240R R1.11/1006

Figure 3.1 Spread-spectrum Waveforms


8/12

Higher values of Rsns will make crosstalk problems worse;try to keep Rsns to 22K or less if possible.

In general try to keep the QT240 close to the electrodes andreduce the adjacency of the sense wiring to ground planesand other signal traces; this will reduce the Cx load, reduceinterference effects, and increase signal gain.

The one and only valid reason to run ground near SNStraces is to provide crosstalk isolation between traces, andthen only on an as-needed basis.

lQ 8 QT240R R1.11/1006


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4 Spec i f ic a t ions

4.1 Absolute Maximum SpecificationsOperating temp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . as designated by suffixStorage temp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -55OC to +105OCVDD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +5.5VMax continuous pin current, any control or drive pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mAShort circuit duration to ground, any pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . infinite

Short circuit duration to VDD

, any pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . infiniteVoltage forced onto any pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to (Vdd + 0.3) Volts

4.2 Recommended Operating ConditionsVDD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +3.9 to +5.0VOperating temperature range, 4.0V - 5.5V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40 - +85COperating frequency, 4.0V - 5.5V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 kHzShort-term supply ripple+noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5mVLong-term supply stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100mVCs (reference capacitors) value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7nF to 47nFCx (SNS load capacitance) maximum value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50pF

4.3 AC SpecificationsVdd = 4.0, Ta = recommended, Cx = 10pF, Cs = 10nF, Rosc = 62K ohms

Fast mode onlybursts6Spread spectrum drive intervalTSS

% of Fosc; Fast mode only;Circuit of Figure 1.2

%7Spread spectrum freq deviationFSSD

Either speed modekHz12010080Acquisition center frequencyFOSC

Fast modeSlow mode

msms

40110

Response time to outputTR

Either speed modecounts400Burst length, each channelNBL

Either speed modems4Burst duration, all channelsTAC

Either speed modes8Clamp durationTPT

Either speed modes2Pulse durationTPC

Fast modeSlow mode

msms

180280

Recalibration time from coldstart or hard reset

TRC

NotesUnitsMaxTypMinDescriptionParameter

4.4 DC SpecificationsVdd = 4.0V, Cx = 10pF, Cs = 10nF, Fosc = 100kHz, Ta = recommended range, unless otherwise noted

bits9Acquisition resolutionAR

OPT1, OPT2, OPT3A1Input leakage currentIIL

OUTn, 1mA sourceVVdd-0.7High output voltageVOH

OUTn, 4mA sinkV0.6Low output voltageVOLSYNCV0.7VddHigh input logic levelVHLS

SYNCV0.3VddLow input logic levelVILS

/RESV0.9VddHigh input logic levelVHLR

/RESV0.4VddLow input logic levelVILR

Required for startup, w/o reset circuitV/s100Supply turn-on slopeVDDS

Fast modeSlow mode, no Sync

mAA

1.590

Supply currentIDD

NotesUnitsMaxTypMinDescriptionParameter

lQ 9 QT240R R1.11/1006


10/12

4.5 Signal ProcessingVdd = 4.0V, Cx = 10pF, Cs = 10nF, Fosc = 100kHz, Ta = recommended range, unless otherwise noted

Option pin selected;Nominal timings; dependent on Fosc, Sync

secs10, 60, infinitePost-detection recalibration timer duration

Nominal timing dependent on Fosc, Syncsecs2Fast negative recalibration timer duration

Nominal timing dependent on Fosc, Syncms/level250Negative drift compensation rate

Nominal timing dependent on Fosc, Syncms/level1,000Positive drift compensation rate

samples6Consensus filter length (Detection integrator)

counts5Fast negative threshold differential

From thresholdcounts2Hysteresis

From signal referencecounts12Threshold differential (positive)

NotesUnitsValueDescription

4.6 Mechanical Dimensions

CE

B

D

C'

Base levelSeating level

F

H

G

A

8080!

0.200.108.04.0H

0.860.6634.026.0G

0.250.1010.04.0F

-0.650--25.59-E

1.851.6573.065.0D

7.496.88295.0271.0C

0.380.2315.09.0C

5.594.98220.0196.0B

8.207.39323.0291.0AMaxTypMinMaxTypMin

Metric (mm)Inches (mils)

SYMBOL

209 mil 20-pin SSOP Dimensions

lQ 10 QT240R R1.11/1006


11/12

4.7 Marking

SSOP-20-40oC to +85oCQT240-ISSG

PackageTemperature RangePart

xxxx = fab code

4.8 Moisture Sensitivity Level (MSL)

IPC/JEDEC J-STD-020C260OCMSL3

SpecificationsPeak Body

TemperatureMSL Rating

lQ 11 QT240R R1.11/1006

QT240-IG

QRG xxxxR

1


12/12

lQ Copyright 2003-2006 QRG Ltd. All rights reserved.Patented and patents pending

Corporate Headquarters1 Mitchell Point

Ensign Way, Hamble SO31 4RFGreat Britain

Tel: +44 (0)23 8056 5600 Fax: +44 (0)23 8045 3939

www.qprox.com

North America651 Holiday Drive Bldg. 5 / 300

Pittsburgh, PA 15220 USATel: 412-391-7367 Fax: 412-291-1015

This device is covered under one or more United States and corresponding international patents. QRG patent numbers can be found onlineat www.qprox.com. Numerous further patents are pending, which may apply to this device or the applications thereof.

The specifications set out in this document are subject to change without notice. All products sold and services supplied by QRG are subjectto our Terms and Conditions of sale and supply of services which are available online at www.qprox.com and are supplied with every orderacknowledgement. QRG trademarks can be found online at www.qprox.com. QRG products are not suitable for medical (including lifesavingequipment), safety or mission critical applications or other similar purposes. Except as expressly set out in QRG's Terms and Conditions, nolicenses to patents or other intellectual property of QRG (express or implied) are granted by QRG in connection with the sale of QRGproducts or provision of QRG services. QRG will not be liable for customer product design and customers are entirely responsible for theirproducts and applications which incorporate QRG's products.

4 key qtouch™ sensor ic

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